The Unity Manual, under the section Publishing Builds says this:

If you are unsure where to save your built game to, consider saving it into the projects root folder. You cannot save the build into the Assets folder.

So, like everyone has said, don't do it. Although the manual doesn't say, I assume saving under the Library folder is frowned upon, too.

You have made builds and placed them in the assets folder?.

This is very bad idea and causes scripts to not compile correctly because of the duplicate dll's placed in the builds.

Remove the build files from your Assets folder and store them somewhere else. This should fix the problem.

Why did you put the build into the assets folder? The assets folder is for the assets that you are using in your game (models, scripts, shaders, textures, sounds and so on) - not for the builds!

You could place your builds either in a separate folder outside of your project folder (that's what I'm doing) or inside your project folder next to the assets folder (which I think is the default - but it doesn't feel "right" to me).